In a construction of a known pivot mechanism, a movable body is hinged to a stationary body that is fixed to a desired position. Further, in a pivot mechanism in which a pivot operation is performed to upwardly pivot a heavy object such as a seat, e.g., in a tumble mechanism for a vehicle seat, a biasing spring may be provided for assisting this operation. Known examples of the biasing spring include a torsion spring member that is formed to a spiral-shape or a torsion bar-shape. The torsion spring member is positioned at a hinge coupling portion. One end of the torsion spring member is connected to a floor-side member mounted on a vehicle floor, which member constitutes the stationary body. The other end of the torsion spring member is connected to a seat-side member mounted on the seat, which member constitutes the movable body. Further, the torsion spring member is assembled in a previously twisted condition, so as to bias the seat in the pivot direction opposite to the gravitational force. Thus, when the seat is upwardly pivoted, a biasing force can be exerted to the seat due to restoration of the torsion spring member. Therefore, it is possible to reduce an operation force required for performing such an upward pivot operation. To the contrary, when the upwardly pivoted seat is downwardly pivoted in a direction of the gravitational force, the above-mentioned biasing force may function as a resistance force. Therefore, when the seat is operated to be downwardly pivoted, such a downward pivot operation is assisted such that the seat can be prevented from being excessively rapidly downwardly pivoted.
Patent Document 1 mentioned below discloses an example of the pivot mechanism as described above.